This invention relates to a method and apparatus for controlling the speeds of at least two motions so as to maintain a prescribed relationship between them. More particularly, the invention relates to a method and apparatus for controlling the speeds of operation of two or more sources of motive power, so as to maintain a prescribed relationship between those speeds and thereby maintain a desired speed relationship between motions driven by those sources.
In a wide variety of industries and applications, drive systems are utilized which require the use of two or more sources of motive power, e.g. electric motors, wherein it is mandatory that a given difference between speeds of the two motors be maintained in order to achieve a given result. The latter difference in speeds is generally described in terms of the ratio of the speed of one motor with respect to the speed of the other motor and is commonly termed "draw ratio".
Generally speaking, the successive motors in a drive system are in effect in a "master-slave" relationship wherein a change in speed in a preceeding motor, the master motor, will bring about a change in speed of a succeeding motor, the slave motor, in order to maintain a prescribed draw ratio. Where a plurality of motors are involved, this relationship continues throughout the succession of motors in the drive system in order to maintain the predetermined draw ratios between each pair of motors. Such systems are commonly used in continuous process manufacturing, such as in the manufacturing of synthetic fibers where a strand or web of material is processed from a supply point through various stages to the finished product. The speeds of the various motors throughout the system will affect the characteristics of the manufactured fiber. In such applications the draw ratios vary over a very narrow range, and it can be anticipated that no motor in the system will have difficulty in meeting the demands placed on it for speed variation. Examples of such systems are to be found in U.S. Pat. Nos. 3,118,097 and 3,447,050.
Multimotor drive systems can find application, as well, in apparatus where one or more motors in the system will have difficulty in meeting the load or speed demand placed on it. An example of such an application, which will be described in greater detail hereinbelow, is an apparatus wherein it is necessary to use more than one motor to drive two or more mechanical members which are used to move an implement through a prescribed path. For example, in a power shovel it may be necessary to provide for the simultaneous rotation and extension of the handle on which the dipper is mounted, and it will be necessary that there be a given relationship between the hoist speed, i.e., the speed of rotation of the dipper handle and the crowd speed, i.e., the speed of the longitudinal movement of the handle in order to effect motion of the dipper through a predetermined digging path. During this operation for a variety of reasons it may be the case that a commanded one of the motors might not be able to run at the commanded speed; for example, in order to maintain the prescribed speed ratio a motor may be commanded to operate at a speed which is beyond its maximum safe design speed. In another situation the motor might not have the capability of delivering the required horsepower at the commanded speed. It also may be the case that an operator in anticipating a given situation might cause a motor being commanded to operate at one speed to operate at a lower speed which would have the effect of taking the speed of the motor outside of the prescribed ratio. In all of these situations if the implement is to continue to move through the desired path, the ratio of the hoist speed to the crowd speed must be maintained and therefore, there must be a change in speed of the commanding motor, as well as the commanded motor.
In other applications involving excavators and the like it may be necessary that the aforementioned speed ratio might have to be varied over a relatively wide range during normal operating cycles. Again, in order to accomplish this it is necessary that the commanding motor be adjusted to take into consideration other influences on the commanded motor.
In the prior art control systems such as those discussed hereinabove in continuous process manufacturing no provision has been made for adjusting the speed of operation of the commanding motor, when for some reason the commanded motor is incapable of or, for example, has been manually adjusted to not respond or is caused to operate at a speed which would change the speed ratio to an undesirable value in the absence of some change in the speed of operation of the commanding motor. Thus, the prior art ratio control systems make no provision for maintaining a given speed ratio between two or more motors when one or more of the commanded motors is unable to respond to cause the system to operate at the prescribed draw ratio.
It is, therefore, an object of this invention to provide means and method for coordinating the speeds of two or more motions so as to establish and maintain a prescribed relationship between those speeds.
Another object of this invention is to provide means and method for coordinating the speeds of two separately driven mechanical motions so that an implement moved by the joint operation of the two motions will follow a prescribed path.
Still another object of this invention is to provide means and method for adjusting two or more sources of motive power to have predetermined speed ratios therebetween where the speed ratios are varied to provide for mechanical motion through a prescribed path.
A further object of this invention is to provide means and method for maintaining a given speed ratio between two sources of motive power, wherein the sources reciprocally command each other, as when a commanded source is unable to reach a speed value communicated by a commanding source so that the former will then in effect command the latter to produce the desired speed ratio.
An additional object of this invention is to provide in a power shovel or the like means for controlling the speed of the motions whereby an implement such as a dipper is moved and the motion of the dipper is maintained in a prescribed path.